NonMendelian Inheritance

There are other patterns of inheritance that do not follow Mendel’s Principles.

Be sure you recognize what is different or what Law is violated by these different patterns.

Mendel’s 3 Laws

1.Law of Dominance

2.Law of Independent Assortment

3.Law of Segregation

1. Co-dominance

Co-dominance=when two alleles are both expressed (neither masks the other)

• Ex = human blood type

• 3 different alleles (= versions of genes coding for one trait) include IA, IB, and i that code for A, B, and “o” blood types.

NOTE: Positive or Negative is another trait and it follows Mendel’s Law of Dominance

Blood Types (continued)Genotype Phenotype• IAi or IAIA Type A blood• IBi or IBIB Type B blood

• ii Type O Blood (the most common)• IAIB Type AB blood (Co-dominant)

Why are there different blood types anyway?

• Because there is a connection between blood type and susceptibility to certain diseases.--AB nearly completely resistant to

Vibrio cholera (a diarrheal) disease

(European descent)

--O slightly more resistant to malaria

(African descent)

2. Incomplete Dominance

• Also known as “blending” b/c neither allele in a pair is fully expressed

• Example seen in Shorthorn Cattle

C = color gene with

alleles possible = CR (red), CW

(white)

• Cross a red bull with a white cow (Punnett Square)

Shorthorn Cattle

CRCW Roan Cattle

3. Linked genes = found on the same chromosome

• Ex: the genes for petal shape & color are linked (C=curved or c=straight / W=white or w=blue); C & w are linked, c & W are linked.

• Cross two heterozygotes (Punnett Square)

• What Mendelian law do these results violate?

Crossing over switches linked genesRead pp. 276-7 in text

4. Sex-linked Traits• Also known as “X-linked”• Mendel’s laws indicate that the parents’

gender should not matter. If you cross a purple flowered plant with a white flowered, it should not matter which is from the male pollen and which is from the female ovule.

• Example 1 = eye color in fruit flies

• Example 2 = Color blindness in humans / normal color seeing is dominant to color blindness but the gene that codes for this trait is linked (found on) to the X chromosome

• X-C = normal color vision• X-c = color blindness• y (doesn’t code for color-seeing)

What letter or number do you see?Light-sensitive opsin

proteins made in the eye & needed for color vision are encoded by a cluster of genes on the X chromosome. Mutations in these genes can lead to an insensitivity to certain colors (like red and green) when seen together (“color vision deficiency”)

• Cross a normal-seeing male with a carrier female.

X-C y x X-C X-c

• How does a colorblind male demonstrate NON-Mendelian inheritance? Hint: How many recessive alleles must an organism normally inherit to express a recessive trait?

5. Multiple Alleles (vs. just two)

• Sometimes a trait is coded for by more than just two alleles

• Example = human blood type has 3 alleles A, B, or O

@ 9q34 = locus of where the

gene is found

6. Polygenic Traits

• More than one set of genes coding for a trait (NOTNOT the same as multiple alleles)

• Eye color is influenced by many genes coding for different kinds of pigment as well as where in the iris those pigments are found (some have been located on chromosomes 15 & 19)

• Think of having 3+ colors from a crayon box and how much and in what order you mix them on a white page dictates the final color.

• http://www.thetech.org/genetics/ask.php?id=203

7. Sex-influenced Traits• Aka, “Gender-influenced”

• Usually influenced by sex hormones like estrogen, testosterone

• Examples include baldness in humans, plumage in birds, horns on cattle

8. Environmentally-influenced

• Color of the Hydrangea flower determined by the pH of the soil

• Acidic soilblue flower

• Basic soilpink flower

• How about “intelligence” in humans?